1. Field of the Invention
The present invention relates to the field of electromagnetic door gaskets and in particular to electromagnetic door seals capable of providing extremely high electromagnetic attenuation over a wide frequency band.
2. Description of the Prior Art
Faraday cages, or electromagnetically shielded rooms have become common and widespread furnishings in manufacturing, research and development facilities throughout the world. Heretofore and continuing to the present day, many of these rooms are manufactured by the user and simply consist of a conductive lining attached to the interior walls, floor and ceiling of the room, such as stapled or nailed metallic screening or sheet. The door or other means of access to the room is similarly lined with a metallic screen to allow user access and then electromagnetic sealing by a simple and conventional mechanical door closure. However, the degree of attenuation provided by casually user-made screened rooms is unacceptable for many applications. In many cases, instrumentation which is being tested within such enclosures is extremely sensitive and erroneous test results will be realized unless there is an overall electromagnetic atteunation, at least 100 db. Cracks within screening joints, thermal expansion between the walls, ceiling and floor, or failure to make metal-to-metal contact around the door seal or any other imperfection in the conductivity easily results in electromagnetic leakages into such rooms which renders the performance of such rooms highly ineffective or entirely useless.
The difficulty of high levels of electromagnetic attenuation, namely 100 db or higher, is even more greatly exacerbated when the room must be shielded not only for a specified domain of frequencies, but must provide a high degree of electromagnetic attentuation over a wide range of frequencies, such as a minimum of 100 db from 10 kHz to 44 GHz. Even in the case of professionally constructed enclosures with carefully designed metal-to-metal contact seals about the door closure, this degree of electromagnetic attenuation across a broad band of frequencies becomes extremely difficult, if not impossible.
Typically, such door seals have included a border of resilient electromagnetic fingers between the door and door jamb which provide for metal-to-metal contact. Examples of such finger stock is illustrated in U.S. Pat. Nos. 3,504,095; 2,825,042; 3,437,735; 3,304,360; 2,783,295; 4,039,741; 3,904,810; and 3,594,490. Generally such seals have been comprised of a length of curved resilient finger stock mated in contact with an opposing flat conductive surface. The degree of attenuation which is provided at various frequencies depends upon the nature and construction of the finger stock, the pressure of metal-to-metal contact achieved, and the overall design of the seal configuration. Performance results are difficult to repeat, and as a practical matter with current designs of seals, the achievement of repeatable sealings is a mystical art.
A single layer of finger stock in contact with an opposing metallic wall has often been found, even under the best of circumstances, to be inadequate. For this reason, the prior art has devised a number of labyrinthian designs such as shown in U.S. Pat. Nos. 3,055,969; 3,589,070; and 4,069,618. Even though the use of labyrinthian designs incorporating multiple stages of finger stock and other metallic separating elements has improved performance, the ability to obtain repeatable sealing pressures has affected the repeatability and reliability of the attenuation.
In a further attempt to solve the problems pertaining to the unreliability of the contact pressures, the prior art has devised labyrinthian seals which include means for resiliently determining the contact pressure of closure such as shown in U.S. Pat. No. 3,296,356.
However, the greater the complexity of the electromagnetic labyrinth which is incorporated into the design, the greater the expense and difficulty of manufacturing. Therefore, in a further attempt to increase electromagnetic seal perfection, the prior art has devised the use of expanded metal to form two-dimensional gasket seals with a multiplicity of contact points between adjacent conductive surfaces, such as shown in U.S. Pat. No. 3,206,536.
Although each of these prior art steps has served to increase electromagnetic seal perfection to a degree, effective attenuation by metallic contact seals have typically been limited to specific frequency spectrums. Normally, such metallic finger stock type seals of any design are effective at the lower frequencies. At higher frequencies, the wavelengths are orders of magnitude smaller and labyrinthian seals of the even the most complex and intricate design appear to the electromagnetic radiation as wide gaping holes in the door seal. To avoid this difficulty, and to avoid the problems inherent in contact pressure, the prior art has devised various non-contact types of seals such as shown in U.S. Pat. Nos. 4,399,316 and 4,514,425, or as used microwave absorbing material from which to fabricate the seal gaskets as shown in U.S. Pat. No. 4,046,983. However, electromagnetically absorptive materials are similarly inherently limited by the frequency dependence of their coefficients of absorption only to the higher frequencies. While such microwave absorbers effectively serve to block or absorb energy in the microwave spectrum and higher, they appear to be translucent or even transparent at the lower frequency radiation.
Therefore, what is needed in some type of electromagnetic absorber which is capable of providing effective and high degrees of energy attenuation from long wave length frequencies ranges to shorter microwave ranges without being subject to the limitations and defects of the prior art.